chore(deps): update actions/cache action to v6 (#4)

This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| [actions/cache](https://github.com/actions/cache) | action | major | `v5` → `v6` |

---

### Release Notes

<details>
<summary>actions/cache (actions/cache)</summary>

### [`v6.0.0`](https://github.com/actions/cache/releases/tag/v6.0.0)

[Compare Source](https://github.com/actions/cache/compare/v6.0.0...v6.0.0)

##### What's Changed

- Update packages, migrate to ESM by [@&#8203;Samirat](https://github.com/Samirat) in [#&#8203;1760](https://github.com/actions/cache/pull/1760)

**Full Changelog**: <https://github.com/actions/cache/compare/v5...v6.0.0>

### [`v6`](https://github.com/actions/cache/compare/v5.0.5...v6.0.0)

[Compare Source](https://github.com/actions/cache/compare/v5.0.5...v6.0.0)

</details>

---

### Configuration

📅 **Schedule**: (UTC)

- Branch creation
  - At any time (no schedule defined)
- Automerge
  - At any time (no schedule defined)

🚦 **Automerge**: Disabled by config. Please merge this manually once you are satisfied.

♻ **Rebasing**: Whenever PR becomes conflicted, or you tick the rebase/retry checkbox.

🔕 **Ignore**: Close this PR and you won't be reminded about this update again.

---

 - [ ] <!-- rebase-check -->If you want to rebase/retry this PR, check this box

---

This PR has been generated by [Mend Renovate](https://github.com/renovatebot/renovate).
<!--renovate-debug:eyJjcmVhdGVkSW5WZXIiOiI0My4yMzIuMCIsInVwZGF0ZWRJblZlciI6IjQzLjIzMi4wIiwidGFyZ2V0QnJhbmNoIjoibWFpbiIsImxhYmVscyI6W119-->

Reviewed-on: #4
Co-authored-by: Renovate Bot <renovate@unbound.se>
Co-committed-by: Renovate Bot <renovate@unbound.se>

.gitea/workflows/ci.yaml

@@ -33,7 +33,7 @@ jobs:
           go install github.com/vladopajic/go-test-coverage/v2@latest
           go-test-coverage --config ./.testcoverage.yml --github-action-output
       - name: Restore baseline coverage
-        uses: actions/cache/restore@v5
+        uses: actions/cache/restore@v6
         with:
           path: coverage-baseline.txt
           key: coverage-baseline-${{ gitea.run_id }}
@@ -88,7 +88,7 @@ jobs:
       - name: Write baseline file
         run: echo "${{ steps.coverage.outputs.total-coverage }}" > coverage-baseline.txt
       - name: Save baseline to cache
-        uses: actions/cache/save@v5
+        uses: actions/cache/save@v6
         with:
           path: coverage-baseline.txt
           key: coverage-baseline-${{ gitea.run_id }}